1. Technical Field
This disclosure relates to the determination of the condition of bone, and in particular to a system and method for monitoring acoustic emissions of bones for detecting, localizing and classifying the condition of such bones.
2. Description of the Related Art
Osteoporosis is a major public health problem which is projected to affect even larger numbers of people as the population continues to age and life spans have been extended. It has been estimated that over 1.5 million osteoporotic fractures occur annually in the United States. As new treatments for osteoporosis become available, one challenge is to improve the ability to identify those people with decreased bone quality and increased risk of osteoporotic fracture, so that effective interventions may be instituted.
Osteoporosis is a disorder currently characterized by a decrease in bone mass and a propensity to develop fractures. Osteoporosis tends to remain asymptomatic, and is often unrecognized until the development of the first fracture. The distal forearm, thoracic and lumbar vertebrae, and the proximal femur are the locations of the most common osteoporotic fractures. The occurrence of any one of these fractures may cause significant morbidity and alteration in lifestyle, including cost of care. A forearm fracture may leave an elderly woman unable to dress and care for herself. Vertebral fractures may be either asymptomatic or may lead to disabling back pain and nerve compression. The morbidity, mortality, and cost associated with hip fractures is substantial. Of those people able to walk at the time of the hip fracture, nearly half will not be able to ambulate without assistance following the fracture. Both the short term and long term costs of caring for hip fracture patients have a tremendous impact on health care expenditures. It has been estimated that the number of hip fracture rates per year in the United States is greater than 250,000, with the direct and indirect costs annually being in the range between $7 billion and $10 billion. Annual estimates for the year 2020, due to the growing size of the elderly population, are between $31 and $62 billion.
Although osteoporosis has been considered to be a disorder primarily affecting post-menopausal women, other large groups of the population are at increased risk for osteoporosis and osteoporotic fractures, including those people receiving steroid and immunosuppressive therapy, and elderly men. Many people who have osteoporosis are asymptomatic until the fracture. One of the major challenges of treatment of osteoporosis is the ability to identify people who are at increased risk of future fracture and to intervene with an effective therapy.
The availability of improved methods of measuring bone mineral density (BMD), such as dual photon absorptiometry (DPA) and dual energy X-ray absorptiometry (DEXA), has greatly improved the ability to identify people who are at increased risk of developing osteoporotic fractures. BMD is an indirect measure of bone strength. However, it has been recognized that a segment of the population will develop osteoporotic fractures despite a normal BMD. In addition, not all patients with decreased BMDs will develop osteoporotic fractures. Other factors, such bone microarchitecture, accumulation of microfractures, and skeletal aging itself are likely to contribute to bone strength and risk of fracture. Current studies have reported ROC curves with values of about 80%.
A non-invasive, safe, and reliable technique to determine bone quality would substantially improve the ability to detect people at risk for the development of fractures, and would be an invaluable technique to monitor response to therapy. Such a technique would need to be better than BMD measurements in the prediction of fracture risk if it is to substantially alter the ability to diagnose osteoporosis and to determine fracture risk.
A primary standard for predicting fracture risks is the accurate measurement of mechanical strength of bone. The goal to non-destructively measure bone strength in vivo has heretofore not been achieved. A scientific method of determining strength of complex structures has been successfully used in the field of non-destructive testing. This method requires the measurement of acoustic emission (AE) signals from stressed materials. Use of AE monitoring (AEM) in bone tissue has so far relied only on the very unsophisticated method of simply counting the number of AE events as a function of time. The approach of treating an AE event as a narrowband process, primarily to minimize measurement noise, and measuring the emission rate during application of stress, has heretofore not been successful when applied in biological systems.
The accompanying sudden, localized change of stress or strain in bone tissue produces wideband AE signals. These changes in stress and their corresponding acoustic emissions are uniquely related to the location and type of changes in bone mass, strength, and architecture, referred herein as bone quality. Generally, almost all bone fractures are preceded by subtle but distinct changes in an AE signature of the bone, and different fatigue modes influence the AE signature differently.
The biological AE signal field may thus be extremely rich with biological information which has not been successfully and effectively exploited for diagnosing osteoporotic conditions.
The term "acoustic emission" is used to describe stress waves emitted by rapid structural changes in material or a solid body. If a force or load applied to the structure causes an inhomogeneity to change in response to the local stress field, a local stress differential will be induced. The stress differential acts as the source of radiation for elastic waves to propagate throughout the structure. Transducers on the surface of the structure will detect these bursts of energy as AE signals, which are naturally wideband phenomena. This mechanism is valid for both non-biological and biological media.
Conventional AE data acquisition methods are essentially scaler measures which utilize narrowband transducers to measure the number of times the root-mean-square (rms) power exceeds a given threshold as a function of time, and is usually referred to as the "AE rate". If the character of the naturally wideband AE signal is preserved and a vector measure is obtained with spatial array processing, then knowledge about the source distribution may be derived from the set of AE signals recorded at multiple observation points, maintaining the relative times-of-arrival. The source of radiation for a given defect will be weighted by the characteristics of the incurred propagation paths and the measuring instrumentation. A complete description of the source distribution therefore requires a reasonably accurate, functional model of the structure architecture and calibration of the instrumentation.
Over the past twenty years, research teams have utilized scaler AE data acquisition and monitoring for a broad range of applications, including both non-biological and biological media. It is now well established that AEM may be effectively employed to: assess soil stability of dams; dikes, retaining walls, and lagoon embankments; and detect and monitor leaks from underground gasoline storage tanks and buried pipelines.
Acoustic emissions are also produced in materials, such as metal and plastic when they are subjected to stress such that they undergo deformation, fracture, or both. AE occurs after "yield", the end of the material's elastic state and the beginning of its plastic state. A strong correlation has been shown to exist between stress versus strain and stress versus AE.
Energy Release Processors (ERP) have been developed for locating flaws and discontinuities in complex piping systems and long runs of buried piping, with the ERP system providing an early warning of significant defect growth. AEM has also revealed the presence of significant cracks in welds of stainless steel steam lines in a thermal power plant. AE patterns have also been successfully measured for fiber-reinforced composites, and composite rocket motor cases. AEM has also been found effective in detecting and pinpointing medium to high pressure leaks in gas distribution systems.
AEM technology has been investigated by several researchers in the 1970's as a diagnostic tool for osteoporosis. It has been shown that the acoustic emission rate from cattle femurs subjected to bending loads is greater for low density specimens as compared to femurs with normal density. These emissions were detected well before the actual bone failure. Recent studies have examined the acoustic emissions from cancellous bone under compression, which also demonstrated that the post-yield acoustic emission rates are significantly higher in both osteoporotic and osteoarthritic bone specimens as compared to normal bone specimens. Results of such studies show a relationship between AE counts versus the applied load and between AE rates versus the applied load.
To date, the major shortcomings of AEM experiments performed on bone tissue are that: (1) they were essentially narrowband measurements not matched to the informational bandwidth of the AE source distribution; (2) the information-bearing attributes of the emitted signals and the structural differences between them were not investigated; and (3) the transient signals normally-occurring in human tissue and in a clinical environment were not evaluated and processed for proper data normalization and signal-to-noise enhancement. The sensitive and reliable data acquisition of incipient bone defects should preserve the spatial and temporal characteristic features in a measurable database that define normal conditions and the features associated with degradation. These characteristic features are referred to as the information-bearing attributes of the AE signature, and are intimately related to the physics and quality of bone tissue.
Other studies have related ultrasonic wave propagation measurements to the structure and anisotropic mechanical properties of osteoporotic and osteopetrotic bone. Bone tissue is a dispersive, anisotropic medium. Acoustic propagation in bone undergoes both geometric and viscoelastic dispersion, with attenuation increasing almost linearly over the frequency range 1-15 kHz. Osteoporosis is characterized by increased porosity or decreased density, while osteopetrosis forms calcified cartilage in bone.
Previous studies of ultrasound have focused on the use of broadband ultrasound attenuation (BUA) for the determination of bone strength. BUA has been found to correlate significantly with BMD as measured by single X-ray absorptiometry; yet there was sufficient variability in the measurements to suggest that BUA provides at least some information about bone in addition to BMD. BUA of the os calcis has been shown to be a better discriminator of hip fracture than DEXA of the lumbar spine or hip. These preliminary observations suggest that ultrasound may provide additional information regarding bone microarchitecture and bone strength.